1. Field of Invention
This invention relates to a projection optical device provided with a projection optical system which projects an image of a predetermined pattern, and to an exposure apparatus which is used in order to transfer a pattern of a mask onto a substrate in order to manufacture various devices such as, for example, a semiconductor device, a liquid crystal display, etc.
2. Description of Related Art
In a lithography process, which is one process used to manufacture a semiconductor device, an exposure apparatus is used in order to transfer and expose a pattern formed on a reticle (or a photomask, etc.) onto a wafer (or a glass plate, etc.) coated by photoresist as a substrate. Various types of exposure apparatus, such as, for example, a step and repeat exposure type (stationary exposure type) projection exposure apparatus such as a stepper, and a step and scan exposure type projection exposure apparatus (scanning exposure apparatus) such as a scanning stepper, can be used.
In exposure apparatus, rigidity of: (i) the stages which move and position a reticle and a wafer, (ii) a support mechanism of the stages, and (iii) a mechanism portion of the support mechanism and the like of a projection optical system, significantly affects the performance capability of the apparatus, such as a vibration control performance capability, an exposure accuracy (overlay accuracy or the like), weight of the mechanism portion, and manufacturing cost of the exposure apparatus. In general, an exposure apparatus having a mechanism portion with high rigidity, while providing a high apparatus performance capability, tends to have a heavy mechanism portion, and a higher manufacturing cost. Furthermore, rigidity of the mechanism portion also is related to the temperature characteristics of the apparatus performance capability, and the stability of the apparatus performance capability corresponding to changes of the apparatus performance capability over time. That is, exposure apparatus having a mechanism portion with high rigidity tend to have good stability with respect to the apparatus performance capability, and excellent temperature characteristics, but depending on the structure of the mechanism portion, there are cases in which the opposite trend occurs. For example, in a mechanism portion, when members with high rigidity are coupled to each other through members having high rigidity, vibration can be easily transmitted, a bi-metal effect is generated at the time of temperature change (if different materials are used for the members), and the temperature characteristics may be deteriorated.
However, as a result of increasing rigidity of the mechanism portion, when the weight of the mechanism portion increases, there also is a possibility of increased construction cost of the device manufacturing factory in which exposure apparatus is installed (in order to deal with the weight of the exposure apparatus). Therefore, conventionally, in order to maintain high rigidity and perform positioning and scanning at a high speed while reducing the apparatus weight, a lightweight material with specific stiffness (value in which rigidity is divided by the weight per unit volume), such as a ceramic, can be used as a material of a part of the members which constitute a stage.
Furthermore, an exposure apparatus also has been proposed in which the stages and the projection optical system are independently supported by parallel link mechanisms, each having a plurality of rods which can expand and contract. This system maintains high rigidity in a necessary portion and lightens the weight of the entire mechanism portion. See, e.g., International Publication No. WO 01/022480.
Thus, in a conventional exposure apparatus, in order to maintain a high device capability with respect to vibration control performance or the like, it is desirable to improve rigidity of a mechanism portion of a support mechanism or the like, while reducing the weight of the mechanism portion. However, among conventional technology, with respect to a method of using a material with specific stiffness and light weight, the material can be used only for a portion of the mechanism portion due to its high manufacturing cost, the material shape, or the like, so the lightening of the entire mechanism portion is not yet significantly improved. In order to further lighten the entire mechanism portion, it is desirable to change the structure itself of the mechanism portion including the support mechanism of the projection optical system.
Meanwhile, in the method that uses parallel link mechanisms, each having a plurality of elongatable rods, it is desirable to further improve the lightening of the mechanism portion, and the control accuracy of a movable portion of a stage. However, there is a possibility that control at the time of scanning and stage positioning also becomes complex because the structure of the mechanism portion becomes complex. Additionally, although the projection optical system can be supported by using the parallel link mechanism, this tends to cause the structure of the mechanism portion to become even more complex. In this regard, in recent exposure apparatus, a thermal distortion amount of the mechanism portion and a fluctuation amount of imaging characteristics of the projection optical system due to the exposure amount of the exposure beam and the surrounding temperature are predicted in advance. According to this prediction result, correction of the imaging characteristics, positioning correction of the reticle and the wafer, or the like is performed during use of the apparatus. However, once the mechanism portion becomes complex, the estimated accuracy of the thermal distortion amount of the mechanism portion and the fluctuation amount of the imaging characteristic deteriorates, and therefore it is possible that the exposure accuracy may deteriorate.
Furthermore, conventionally, in order to control the fluctuation amount of the imaging characteristic of the projection optical system due to the temperature fluctuation, a cooling liquid is supplied to the surrounding of the projection optical system. In this case, in order to increase the vibration control performance capability of the exposure apparatus, it is desirable that the vibration of the motive force which supplies the cooling liquid should be controlled.